1. Field of the Invention
The present invention relates to a front-wheel steering system and, more particularly, to a front-wheel steering system for controlling the steering angle of the front wheels of a vehicle using an electric motor, in accordance with the running condition of a motor vehicle.
2. Description of the Prior Art
There has been proposed an electric power steering system assisting a steering power by using an electric motor as disclosed in, for example, Laid-Open Japanese Patent Application No. 62-218268.
In this steering system, a steering shaft is connected with a pinion shaft, and the pinion shaft is in mesh with a rack tooth section of a rack shaft, thereby mechanically connecting a steering wheel to road wheels. Also in mesh with the rack shaft is a pinion shaft which is driven by the electric motor, assisting the steering power by the electric motor. The driving torque of this motor is transmitted to the pinion shaft through the reduction gear and an electromagnetic clutch. Connection and disconnection of the electromagnetic clutch are controlled by means of a control unit.
In the above-mentioned prior-art steering system, the electromagnetic clutch is disconnected at a high vehicle speed or in case of a trouble present in the control unit, allowing a vehicle driver to manually operate the steering system. The power-assistance ratio of the electric motor to the muscular effect of the driver for steering is typically around 10:1, and therefore the driving force of the electric motor is greater than the muscular force of the driver. If, therefore, the electric motor malfunctions, it is very difficult or impossible for the driver to operate the steering system. The prior-art steering system, therefore, is provided with an electromagnetic clutch for a fail-safe steering purpose, by which the electric motor is disconnected from the steering mechanism in the event of malfunction of the motor.
However, since the electromagnetic clutch is electrically driven by the control unit, it still has a reliability problem.
In the prior-art steering system described above, since the steering wheel and the front wheels are mechanically connected, it is imperative to depend on the manipulation of the steering wheel when controlling the steering angle of the front wheels. Accordingly, it is impossible to meet a demand to improve the driving stability of the vehicle by determining the steering angle of the front wheels in accordance with the driving condition of the vehicle.